Recently, in the field of electro-optical information technology, especially the techniques related to a short wavelength laser source useful in the digital optical head are widely discussed. In 1962, Prof Bleombergen et al. at Harvard University (U.S.A.) made theoretical calculation and proposed a quasi-phase matching (to be abbreviated as QPM here below) algorithm of converting a fundamental wave to a second harmonic wave by setting pitches and modulating the polarities of the second order non-linear coefficients to obtain a periodical domain reversal structure. Such knowledge is disclosed in, for example, Phys. Rev., vol. 127, No. 6, 1918 (1962) and U.S. Pat. No. 3,384,433 (1968). FIG. 1 shows a general periodic domain reversal structure, which describes the relation of distance and second harmonic generation (to be abbreviated as SHG here below) and the comparison under quasi-phase matching and phase mismatching conditions. In such a method, the problem of phase mismatching defined as the wave vector difference between the fundamental wave and the nonlinear optical wave in the propagation direction can be overcome by the addition of a wave vector of 2.pi./.LAMBDA. from the periodically poled structure with a period of .LAMBDA..
The QPM method has advantages over the commonly used birefringence phase matching method in conventional non-linear optical crystals in that the former uses the maximized second order non-linear coefficient d.sub.33 to ensure that all three waves involved in the nonlinear frequency conversion process are polarized in the same direction. Accordingly, the walk-off angle problem arisen from the angular deviation between the fundamental and nonlinear waves of different polarization in the conventional birefringence phase matching method can therefore be avoided.
So far, the QPM structures on the non-linear optical crystals have been realized by either diffusion methods or crystal growth methods. The former include a method for domain reversal by making use of proton exchange or metal-induced diffusion process, and the latter a method of using temperature gradient during the process of crystal growth to modulate of the direction of spontaneous polarization. Such knowledge is disclosed in, for example, IEEE J. Quantum Electronics, vol. 33, No. 10, p. 1673 (1997) by Webjorn el al.. However, due to the crystal directional dependence of the diffusion process, the use of diffusion method often results in a shallow domain reversed region of triangular profiles. The latter deviates strongly from the idea case of 180.degree. domain reversal as shown in FIG. 1 of the prior art. In addition, the irregularities associated with the diffusion process that occurs at the domain boundaries can greatly reduce the second order nonlinear optical coefficient. Such knowledge is disclosed in, for example, by Veng et al. in Appl. Phys. Lett. Vol. 69, No. 16, p. 2333 (1996), and by Webjorn et al. in IEEE Photonics Technol. Lett., vol. 1, No. 10, p. 316 (1989). A combination of these effects leads to a substantial reduction in the efficiency of nonlinear frequency conversion using the diffusion method. On the other hand, the use of temperature control method in achieving periodical poling of nonlinear optical crystals is limited to small sample size. See, for example, Magel et al. in Appl. Phys. Lett., vol. 56, No. 2, p. 108 (1990).
The other related prior arts will be briefly reviewed hereinafter: to begin with, in 1993, Yamada et al. with Sony Corp. (Tokyo, JP) reported the formation of bulk type of periodically poled lithium niobate (to be abbreviated as PPLN here below) QPM structures with a small period of .LAMBDA.=2L.sub.c =5.6 .mu.m on the thin lithium niobate substrate of two hundred micrometers (200 .mu.m) in thickness by applying a high pulse voltage across the electrodes, where L.sub.c denotes the coherence length defined as L.sub.c =.pi./(k.sub.2.omega. -2k.sub..omega.). They further claimed that the conversion efficiency of the second harmonic generation (SHG) waveguide is 600%/W-cm.sup.2, much higher than that in prior arts. However, since the electric field necessary for domain reversal is as high as 24 kV/mm, the experiment can only be processed in oil or under high vacuum condition, to be more precisely, a pressure lower than 10.sup.-5 torr to avoid dielectric breakdown. M. Yamada and T. Yamaguchi with Sony Corp. hold U.S. Pat. Nos. 5,526,173 (1996), 5,249,250 (1993), and 5,193,023 (1993) for their research in bulk periodic poling of lithium niobate.
Sony Corp. holds U.S. Pat. No. 5,526,173 (1996) "Method of local domain control on non-linear optical crystals" for their design on the plural form of electrodes on the periodically poled lithium niobate (PPLN) structures to reduce the fringe field effects. The latter arises from the dielectric discontinuity in preparing the PPLN samples and can lead to substantial lateral domain motion in the high field poling process. As a result, a high fidelity of 50-50% duty cycle in the ideal quasi-phase matching (QPM) structures of small period for short wavelength conversion becomes difficult to achieve.
Please refer to FIGS. 2A to 2C, in which the fringe field effect is described. As shown in FIG. 2A, symbol 20 denotes the lithium niobate substrate, on which is coated with a layer of photoresist 10. FIG. 2B and FIG. 2C represent the electric field distribution along the crystal+z axis and the x-axis, respectively, as a function of distance away from the top surface of the electrode. In the figures, the x-coordinate and y-coordinate represents, respectively, the normalized distance (x/.LAMBDA..sub.c), and normalized field strength (E/E.sub.applied), where .LAMBDA..sub.c denotes the period of the QPM structure and E.sub.applied the applied poling field strength. Such knowledge is disclosed in, for example, by Rosenman et al., in Appi. Phys. Lett. vol. 73, No. 7, p. 865 (1998).
From the principle of the electromagnetic theory, it is known that the discontinuity of dielectric distribution such as seen in FIG. 2 of the prior art due to the coating of photo-resist and evaporation of Al metal in preparing the PPLN samples, would cause the discontinuity of electric field (i.e. the so-called fringe field effect) at the edges of the electrodes. Since the fringe field at the edges of the electrodes is much higher than the corresponding coercive field required for achieving a 180.degree. polarization switching, the domain reversal process will hence proceed along the crystal z- and x- axis. The former procedure is known to result in a fast 180.degree. reversed domain formed in the crystal z direction, while the latter can cause a lateral 180.degree. domain motion not restricted to the areas defined by the electrodes. A loss of 50-50% duty cycle fidelity in the QPM structures due to the fringe field effects can therefore generates phase mismatching and thus reduce the nonlinear frequency conversion efficiency.
In addition to control the lateral 180.degree. domain motion, another important consideration in fabricating periodically poled QPM structures is the capability to stabilize the reversed domain structure. In the conventional electric poling process on congruent grown ferro-electric crystals, the switching off of the high voltage source at the end of the poling process can introduce a charge back-flow current according to i=C dv/dt. The latter can cause a relaxation of the inverted polarization back to its original direction such that the periodically poled structures previously formed during the high voltage switching on period can thereby be partially washed out. In the U.S. Pat. No. 5,652,674 "Method for manufacturing domain-inverted region, optical wavelength conversion device utilizing such domain-inverted region and method for fabricating such device", K. Mizuuchi and K. Yamumoto with Matsushita Electric Industral Co., Ltd. (Kadoma, Japan) applied a combination of DC and pulse voltage to reduce such a charge back flow process on thin polished lithium tantalate (LiTaO.sub.3) substrate of two hundred micrometers (200 .mu.m) in thickness.
In the U.S. Pat. No. 5,838,486 (1998) "Optical wavelength conversion element, method of manufacturing the same and optical wavelength conversion module", Sonoda et al. with Fuji Photo Film Co., Ltd. (Kanagawa-ken, Japan) proposed a method in which a Y-cut magnesium oxide doped lithium niobate (MgO:LiNbO.sub.3) substrate tilted at an angle .theta. larger than zero degree and smaller than ninety degrees is used to fabricate shallow type of reversed domain structure of QPM structures with reversed domain of two to three micrometers (2.about.3 .mu.m) in depth. Magnesium oxide doped lithium niobate has advantages over undoped lithium niobate in that the optical damage resistance of the former is at least ten times greater than that of the latter, and the non-linear coefficient d.sub.33 of the former is larger than that of the latter. Such knowledge is disclosed in, for example, Nakamura et al. in Jpn. J. Appl. Phys., vol. 38, No. 5A, p. L512 (1999).
Recent invention on the technique of fabricating PPLN also- includes the U.S. Pat. No. 5,800,767 (1998) "Electric field domain patterning", by Byer et al. with Stanford University (Stanford, Calif.) in which a liquid electrode method is proposed for periodic poling of lithium niobate.
Despite vast interest in applying periodically poled lithium niobate (PPLN) and periodically poled lithium tantalate (PPLT) for nonlinear frequency generation, great challenge remains in reducing the coercive field strength, which is generally larger than 21 kV/mm, such that fine 180.degree. domain reversed QPM structures with smaller period can be fabricated on PPLN and PPLT. In the above mentioned technology, substrates made of congruent grown lithium niobate or lithium tantalate have become the crystal of choice due to the well established crystal growth techniques. However, it is known that the congruent grown lithium niobate and lithium tantalate suffer from the crystal non-stoichiometry such that an large internal field on the order of 2.5 kV/mm associated with the anti-site or vacancy point defects exists along the crystal +Z axis. Such knowledge is disclosed in, for example, J. Phys. Chem. Solids, vol. 52, No. 2, p. 185 (1991) by Schirmer et al., and in Appl. Phys. Lett. vol. 72, No. 16, p. 1981 (1998) by Gopalan et al.
Recently, a promising method of reducing the coercive and internal field on lithium niobate and lithium tantalate has been discovered by Kitamura et al. They report a use of double crucible Czochralski method to grow stoichiometric crystals, and the measured coercive fields are reduced to 4.5 kV/mm and 1.5 kV/mm, and the internal field reduced to less than 0.5 kV/mm, respectively, for the stoichiometric grown lithium niobate and tantalate crystals. Such knowledge is disclosed in, for example, Appl. Phys. Lett., vol. 73, No. 21, p. 3073 (1998), and in Appl. Phys. Lett. vol. 72, No. 16, p. 1981 (1998).